This invention relates to an improved temperature sensor for use with a solid disk automatic surface unit for an electric range or cooktop.
The use of automatic temperature controls for electric range surface units is well known. Generally, a utensil temperature sensor is mounted in the center of the surface unit to physically contact the bottom of the cooking utensil being heated. The utensil temperature is sensed by a sensing element such as a thermistor or thermocouple and the resulting signal is used by the automatic temperature control circuitry to maintain a desired cooking temperature as set by the user. An example of such control circuitry is illustrated in U.S. Pat. No. 4,493,980 which describes operating modes for boiling and frying. In the boiling mode, the temperature information may be used to provide rapid heat up, detect the boil point and maintain selected approximate boil rate. Examples of boil point detection arrangements may be found in U.S. Pat. Nos. 4,465,228 and 4,665,292.
The ability of the sensor to accurately sense and follow temperature variations in the cooking utensil is critical to maintaining the desired cooking temperature. When operating in a boil mode in which power is reduced upon detection of reaching the boil point, accuracy during the heat up phase is particularly critical as the premature detection of boil point may result in an unnecessarily prolonged heat up period. One factor adversely affecting accuracy is the exposure of the sensor to heat from the surface unit in addition to heat from the utensil. In commonly used sheathed surface heating units the sensor is spaced somewhat from the surrounding heating unit. Thus, the primary heat transfer mechanism is radiation from the sheathed element rather than conduction. One approach found to be effective in protecting against such radiation is disclosed in U.S. Pat. No. 4,241,289.
However, solid disk surface units are finding increasing popularity in this country. One particular advantage is that the solid disk unit provides a closed surface, lending itself to easier cleanability. In order to provide a solid disk automatic surface unit which retains the closed surface advantage, the sensor assembly must substantially fill the central opening provided in the solid disk to accommodate the sensor. An example of one such surface unit equipped with an electromechanical sensor and control arrangement is disclosed in U.S. Pat. No. 4,330,701. The sensor i this arrangement uses fluid expansion in a capillary tube to sense utensil temperature. The sensor head is a sheet metal dish which extends across the central opening in the surface unit. Since the sensor is not spaced from the surface unit as it is in sheathed heating units a potential problem with such an arrangement results from the sensor head being additionally heated directly by the surrounding surface unit. In typical fry mode operations in which the user selects a desired steady state temperature for the utensil, the temperature variations need not be accurately followed in order to satisfactorily maintain the selected nominal temperature. Thus, the affect of this heat from the surface unit is tolerable. However, for applications in which following variations in temperature accurately is more critical, such as with control systems which provide the above-described boil mode in which the surface unit is driven at full power prior to boil point detection to provide faster response, and relatively low power thereafter, heating of the sensor directly by the surface unit can seriously impair performance.
Therefore, a need exists for and it is a primary object of this invention to provide a sensor assembly for use in a solid disk surface unit, which retains the cleanability advantages associated with a closed surface while preventing the detrimental effects of direct heating of the sensor by the surrounding surface unit.